1. Field of the Invention
The present invention relates to a temperature sensor for measuring the temperature of a fluid, such as exhaust gas from an engine. More particularly, the invention relates to a temperature sensor having a temperature sensor element, such as a thermistor, disposed in a front end portion of a metal tube having a closed front end (a closed-bottomed tube or cap), and which is attached to an exhaust manifold (an exhaust gas pipe) such that the front end of the tube is exposed to exhaust gas, and is favorably adapted to measure the temperature of the exhaust gas.
2. Description of the Related Art
Conventionally, a temperature sensor (hereinafter, also referred to as a sensor) of this type has been proposed as embodying various structures (refer to, for example, Patent Document 1). The temperature sensor disclosed in Patent Document 1 has the following structure: a temperature sensor element (hereinafter, also referred to as a sensor element or an element) coated with glass is disposed at the front end of an insulation sheath formed of an insulative material; electrodes (electrode wires) of the element and the core wires (lead wires) connected to the respective electrodes extending through the insulation sheath; and a tube made of metal and having a closed front end accommodating these components such that the element is disposed in a front end portion of the tube. In the temperature sensor, in order to improve accuracy of measurement by enhancing responsiveness and temperature sensitivity, the front end of the element is in direct contact with the tube which protects the element, thereby enhancing heat transfer to the element. Responsiveness and temperature sensitivity may deteriorate as a result of heat transfer (heat release) through the tube, for example, immediately after cold start of the engine. In fabricating the temperature sensor, cement (heat resistant cement) which serves as a filler is charged into a space between the inner circumferential surface of a portion of the tube located toward the front end of the tube and the outer circumferential surface of a portion of the insulation sheath located toward the front end of the insulation sheath and is then dried and set, thereby fixing the element and the insulation sheath within the tube. In this manner, a state of contact is maintained between the element and the tube.
In the temperature sensor, a seal member (a bushing or a cap) made of rubber is fixed in a rear end portion of the tube for establishing gastightness and liquid-tightness, and lead wires electrically connected to the respective electrode wires extend through the seal member in the front-rear direction and extend outward from the seal member. Further, in the temperature sensor, a mounting metal fitting having external threads formed on its outer circumferential surface is externally fitted to the tube. The mounting metal fitting is threadingly mounted to an exhaust manifold or the like (hereinafter, also referred to as an exhaust manifold), whereby the temperature sensor is put into use.
Meanwhile, in the temperature sensor, as mentioned above, the sensor element and the insulation sheath are only fixed by means of cement charged into a portion of the tube located toward the front end of the tube. Further, only a portion of the insulation sheath located toward the front end of the insulation sheath is fixed, whereas a portion of the insulation sheath located toward the rear end of the insulation sheath is not fixed. Specifically, the lead wires (output lead wires) project from the rear end of the insulation sheath, and the lead wires alone are retained by extending the lead wires through a seal member disposed and fixed in a rear end portion of the tube. The rear end of the insulation sheath is not actively fixed, but is in a state of having a nearly free end.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H07-140012
3. Problems to be Solved by the Invention
As mentioned above, in the temperature sensor described in Patent Document 1, the insulation sheath having the sensor element disposed at its front end is fixed within the tube only in such manner that a portion of the insulation sheath located toward the front end of the insulation sheath is fixed by means of cement. Thus, upon vibration or impact, the rear end of the insulation sheath is likely to, for example, laterally oscillate about the front end of the insulation sheath. Such lateral oscillation is likely to cause cracking in the cement which fixes the element as well as a portion of the insulation sheath located toward the front end of the insulation sheath contained in a portion of the tube located toward the front end of the tube. Also, as a result of alternating exposure to high temperature and low temperature in the course of actual use, associated thermal variations may cause cracking in the cement. As a result, in some cases, the cement is broken into pieces starting from such cracks. In such a case, a problem of deterioration in responsiveness or temperature sensitivity arises for the following reason: since the cement breaks up and scatters within the tube, the cement loses its function of fixing the element and the insulation sheath, so that the element separates from the front end of the tube or moves rearward.
Also, in some cases, the following problem arises: in using the temperature sensor mounted to the exhaust manifold, vibration or impact imposed on the temperature sensor causes the insulation sheath to oscillate at its rear end. As a result, stress is repeatedly imposed on the core wires projecting from the rear end of the insulation sheath, and associated metal fatigue causes the core wire(s) to break. Particularly, in the case where the cement loses its function of fixing the element and the insulation sheath, the insulation sheath is likely to move. Therefore, the risk of breaking the core wires is high.